Electrochemical oxygen separator cell

ABSTRACT

An electrochemical oxygen separator cell including an electrode based on lanthanum strontium manganese oxide or lanthanum strontium cobalt iron oxide; and an electrolyte membrane of doped ceria.

The present invention relates to electrochemical cells for separating oxygen from other gases to produce an oxygen-enriched stream or an oxygen-depleted stream.

A large number of commercial processes needs oxygen, oxygen-enriched or oxygen-depleted stream. Examples of industrial processes requiring oxygen enriched stream uses include glass production, petrochemical industry, paper industry, metallurgical industry, aerospace and medical applications. Oxygen depleted streams can be advantageous for lowering the emission of nitrogen oxides (NO_(x)) by diesel engines.

There are many differing methods and apparatus used for separating oxygen from other fluids, such as cryogenic cycles, non-cryogenic air separation plants, including the use of molecular sieves.

One of such methods uses an electrochemical process where an oxygen-containing gaseous mixture, such as air, is fed to one side of a ceramic membrane with an electrical potential applied across the membrane. The oxygen molecules are reduced to oxygen ions at the interface between the cathode and the electrolyte membrane and the oxygen ions can selectively pass through the electrolyte.

After passing through the electrolyte, a further reaction takes place at the interface between the electrolyte and the anode where the oxygen ions are oxidized to reform oxygen molecules. By using of particular electrolyte membranes, only the oxygen ions are allowed to pass through the cell and thus the overall process is very selective for producing a stream with high concentration of oxygen.

In further detail, when an electrical potential is applied across an oxygen ion electrolyte membrane via electrodes, oxygen is dissociated and reduced at the cathode according to the following reaction O₂→4e ⁻→2O⁻²

Oxygen ions migrate through the electrolyte, and are oxidised and recombined at the anode to produce oxygen. An external electrical connection allows the transfer of electrons from the anode to the cathode. The flux of oxygen produced by an electrically driven force is directly proportional to the current passing through the electrolyte membrane according to the Faraday law, ${{mols}\quad O_{2}} = \frac{I \times t}{4 \times F}$ wherein I is the electrical current (A),

-   -   t is time (sec),     -   F is the Faraday constant (i.e. 96485.3 C/eq) and     -   4 is the number of electrons exchanged in the electrochemical         reaction 2O⁻²→4e⁻+O₂, eq/mol.

This means that the flux of oxygen for an applied potential is governed by the electrochemical resistance of the cell (the sum of the electrolyte and electrode polarisation resistance). The O₂ flux can be increased by either raising the potential of the electrochemical cell or reducing the resistance of the membrane.

U.S. Pat. No. 5,021,137 (in the name of Ceramatec Inc.) relates to a ceramic solid electrolyte based electrochemical oxygen concentrator cell. The cell is based on a doped cerium oxide ceramic solid electrolyte and lanthanum strontium cobaltite (LSCO) ceramic electrodes. Preferably, cerium oxide is doped with calcium oxide, strontium oxide or yttrium oxide. This cell exhibits a current density of 450 mA/cm² at 800° C. and 1.0V dc operating voltage.

D. Waller et al. Steele, Electrochemical Society Proceedings Vol. 95-24 (1997) pp. 48-64 disclose oxygen separation using dense gadolinia doped ceria membranes. More specifically, Ce_(0.9)Gd_(0.1)O_(1.95) (ceria doped with 10% of gadolinia, hereinafter referred to as CGO-10) as electrolyte is screen printed or painted with lanthanum strontium cobalt iron oxide (LSFCO) as electrodes. The X-ray diffraction (XRD) data of the LSFCO electrodes show a polyphase pattern. The current density provided by this construction is of almost 350 mA/cm² at 800° C. and 0.6V dc operating voltage.

This paper reports that for achieving a production of 1 ml of oxygen per minute and per cm², it is necessary a separator showing a current density of at least 287 mA/cm². It is established that the electrode resistance is the predominant factor in limiting the oxygen flux through the cell. Reducing the electrode resistance is the key factor in increasing the performance of the cell. The electrolyte resistance constitutes only a small proportion of the overall resistance of the cell; therefore the thickness of the electrolyte may be increased (e.g. from 100 to 250 μm) to improve the mechanical strength of the cell without giving rise to a large increase in the overall resistance of the cell.

Applicant faced the problem of providing an electrochemical oxygen separator cell with higher performance, in term of current density and, as a consequence, of oxygen separation, with respect to those known in the art.

This problem is solved by providing an electrochemical oxygen separator cell with a specific combination of material for electrolyte membrane and electrodes which yields surprisingly high performances also in the presence of a cell architecture wherein the supporting element is one of the electrode, thus having a thickness greater than that of the electrolyte membrane.

Therefore the present invention relates to an electrochemical oxygen separator cell including

-   -   a cathode comprising a material selected from lanthanum         strontium manganese oxide/doped ceria in a ratio ranging between         about 85:15 and about 75:25 by weight; and lanthanum strontium         cobalt iron oxide;     -   an electrolyte membrane comprising ceria doped from about 15 to         about 25% by mole;     -   an anode comprising a material selected from lanthanum strontium         manganese oxide/doped ceria in a ratio ranging between about         85:15 and about 75:25 by weight; and lanthanum strontium cobalt         iron oxide.

Unless otherwise indicated, in the following of the description lanthanum strontium cobalt iron oxide will be referred to as LSFCO, and lanthanum strontium manganese oxide will be referred to as LSMO.

In the following of the description, cathode and anode could also be referred to as “electrode”.

Examples of doped ceria useful in the present invention are gadolinia doped ceria and samaria doped ceria.

The doped ceria is used as electrolyte membrane material is preferably doped in an amount of about 20% by mole. Preferred in this connection is Ce_(0.8)Gd_(0.2)O_(1.90) (hereinafter referred to as CGO-20).

Cathode and anode of the present invention can have the same or different composition and morphology.

Preferably, lanthanum strontium manganese oxide/doped ceria ratio is from about 80:20 to about 70:30 by weight.

Preferably La_(1-x)Sr_(x)MnO_(3-δ) is La_(0.8)Sr_(0.2)MnO₃ (hereinafter referred to as LSMO-80).

Preferred material for electrode is La_(1-x)Sr_(x)Fe_(1-y)Co_(y)O_(3-δ), wherein x and y are independently equal to a value comprised between 0 and 1 included and 8 is from stoichiometry, more preferably La_(0.6)Sr_(0.4)Fe_(0.8)Co_(0.2)O_(3-δ) (hereinafter referred to as LSFCO-80). Preferably, LSFCO is in single phase belonging to the perovskite family, i.e. a group of compounds of the general formula ABX₃ with X most frequently oxygen. Optionally, LSFCO can be added with doped ceria.

Both cathode and anode preferably show a porosity at least of about 20% (measured by SEM).

In a preferred embodiment, electrochemical oxygen separator cell of the invention shows one electrode (supporting electrode) being substantially thicker than the electrolyte membrane. Preferably the supporting electrode is the anode.

For example, the supporting electrode shows a thickness of about 100-600 μm. For example, the electrolyte membrane has a thickness ranging between about 0.5 μm and about 20 μm, more preferably between about 2 μm and about 10 μm.

In another aspect the present invention relates to an apparatus comprising the electrochemical oxygen separator cell disclosed above. Said apparatus may be an engine for vehicle transportation, electrochemical reactors for synthesis of syn-gas from hydrocarbons, alcohols, acids from methane, high purity oxygen supplier for medical applications and for petrochemical, aerospace and metallurgy industries.

One of the applications for the electrochemical oxygen separator cell of the invention is related to the reduction of the contaminant emission in diesel engine, in particular NO_(x) exhausts and particulates. In this connection, the electrochemical oxygen separator cell finds application in two ways, i.e. enriching or depleting the oxygen flow to the engine. The enriching of oxygen flow to the engine reduces particulate emissions, especially at cold start, increases engine power output, and allows the use of lowergrade fuels.

The depleting of oxygen flow to the engine reduces NOx emissions without the problems caused by exhaust gas re-circulation (engine wear, oil contamination), and eliminates the need for a heat exchanger to cool exhaust gases before recirculation.

The present invention will be further illustrated by means of the following examples and figures, wherein:

FIG. 1 illustrates an XRD of the CGO powders as prepared in example 1 treated at different temperatures;

FIG. 2 schematically shows the polarization experimental setup;

FIG. 3 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 1;

FIG. 4 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 2;

FIG. 5 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 3;

FIG. 6 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 4;

FIG. 7 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 5;

FIG. 8 illustrates the polarization measurement of the electrochemical oxygen separator cell of Example 6.

An electrochemical oxygen separator cell according to the schematic drawing of FIG. 2 comprises an anode (1), an electrolyte membrane (2), a cathode (3), and metal contacts (4) for the connection to electric circuits.

EXAMPLE 1 LSMO/CGO-CGO-LSMO/CGO (Symmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80

-   -   Thickness: ˜20 μm.         Electrolyte membrane: Composition: CGO-20     -   Thickness: 450 μm         Anode: Composition: 30% wt. of CGO-20+70% wt. of LSMO-80     -   Thickness: ˜20 μm.         a) CGO-20 Powder Synthesis

A solution of 12.6 g pf oxalic acid (Aldrich 99.999%) in 250 ml of H₂O was brought to pH=6.5 with NaOH (0.1M) (Aldrich). 8.0 g. of Ce(NO₃)₃.6H₂O (Aldrich 99.99%) and 2.078 g Gd(NO₃)₃.6H₂O (Aldrich 99.99%) were added to 50 ml of H₂O and stirred up to complete dissolution. This cationic solution was dropwise added to the oxalic solution to give a ratio 1 mol Ce³⁺:˜6 mol H₂C₂O₄ and 1 mol Gd³⁺:˜6 mol H₂C₂O₄. The formed precipitate was filtered, thrice washed with water and dried at 100° C. for 4 hours. The pH of the water used for washing was up to 6.5. The dried powder was crashed and crystallised at 700° C. for 4 h. A CGO-20 nanopowder (4 g) was obtained. The nanopowder has a particle size of 26 nm measured from the XRD pattern (FIG. 1) by line broadening measurements using the Scherrer equation K·l/β·Cos θ

wherein K is the shape factor of the average crystallite;

l is the wavelength,

β (rad) is the full width at half maximum of an individual peak, and

θ (rad) is the peak position (2θ/2).

b) CGO-20 Electrolyte Membrane Preparation.

-   -   CGO-20 powder of point a) was thermally treated at 1050° C. for         1 h, then uniaxially pressed at 300 MPa, and the resulting         pellet was thermally treated at 1550° C. for 3 hours to give a         membrane about 450 μm thick, with a relative density         (experimental density/theoretical density) higher than 95%.         c) Electrode Preparation     -   30 mg of CGO powders of point a) were mixed in an agata mortar         with 70 mg of LSMO-80 (Praxair 99.9%), and added with 1.5 ml of         isopropanol (Carlo Erba) in a ultrasonic bath for 10 min to give         a slurry. Said slurry was painted on both side of the         electrolyte membrane of point b), then dried at 150° C. for 1 h         in air conditions. The electrode and electrode/membrane         interface were sintered at 1110° C. for 2 h in air conditions.         d) Polarisation Measurement.

The polarisation measurement was carried out by potentiodynamic measurement [by applying a constant voltage (V) and measuring the current (I)] in a four electrode cell-configuration (FIG. 2). The measurements were carried out by an AUTOLAB Ecochemie potentiostat/galvanostat and impedance analyzer, at 800° C. The results are set forth in FIG. 3. A current density of 1.0 A/cm² was observed at 1.4V do operating voltage.

EXAMPLE 2 LSCFO-CGO-LSCFO (Symmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: LSCFO-80

-   -   Thickness: ˜20 μm.         Electrolyte membrane: Composition: CGO-20     -   Thickness: 450 μm         Anode: Composition: LSCFO-80     -   Thickness: ˜20 μm.

CGO-20 powder and membrane were prepared as from steps a-b) of Example 1.

a) Electrode Preparation

-   -   LSFCO-80 powder (100 mg; single perovskite phase, primary         particle size 9 nm, BET surface area: 4.12 m²/g, Praxair) was         homogenised in an agata mortar (with 1.5 ml of isopropanol         (Carlo Erba) in an ultrasonic bath for 10 min, to provide a         slurry. Both sides of the CGO-20 electrolyte membrane were         painted with said slurrys and dried at 150° C. for 1 hour in air         conditions, then the electrode and electrode/membrane interface         were sintered at 1110° C. for 2 hours in air conditions.         b) Polarisation Measurement.

The cell evaluation was carried out as described in Example 1, d). The results are set forth in FIG. 4. A current density of 1.2 A/cm² was observed at 0.6V dc operating voltage.

EXAMPLE 3 LSMO/CGO-CGO-LSMO/CGO (Asymmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80

-   -   Thickness: ˜20 μm.         Electrolyte membrane: Composition: CGO-20     -   Thickness: 8 μm         Anode: Composition: 30% wt. of CGO-20+70% wt. of LSMO-80     -   Thickness: ˜500 μm.

CGO-20 powder was prepared as from step a) of Example 1.

a) Anode Preparation

-   -   0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) were mixed in         an agata mortar. The mixed powders were are pressed in a         cylindrical shape (φ=16 mm, d=1 mm) under uniaxial pressure of         200 MPa for 30 min. The resulting green pellet was sintered at         900° C. for 2 hours.         b) Electrolyte Membrane Preparation     -   CGO-20 powder (1 g) was mixed with ethanol (2 ml) in a ball mill         for 4 hours to give a slurry. Said slurry (0.5 g 20 ml of         ethanol, and the resulting suspension was placed for 4 hours in         an ultrasonic bath. The resulting solution was sprayed by an         aerograph device on the anode (supporting electrode) of step a)         for 3 min, then sintered at 1300° C. for 6 hours.         c) Cathode Preparation     -   0.21 g of CGO-20, made in example 1, a) were mixed in agata         mortar with 0.49 g of LSMO-80 (Praxair 99.9%), by adding 2 ml of         ethanol, for 4 hours, to give a slurry. Said slurry (0.5 g) was         added with 20 ml of ethanol, and the resulting suspension was         placed for 4 hours in an ultrasonic bath. The resulting solution         was sprayed by an aerograph device on the electrolyte membrane         of step b) supported by the anode, for 3 min, then sintered at         1100° C. for 1 hour.         d) Polarisation Measurement.

The cell evaluation was carried out as described in Example 1, d). The results are set forth in FIG. 5. A current density of 3 A/cm² was observed at 0.8 V dc operating voltage.

EXAMPLE 4 LSCFO-CGO-LSCFO/CGO (Asymmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: LSCFO-80

-   -   Thickness: ˜20 μm         Electrolyte membrane: Composition: CGO-20     -   Thickness: 8 μm         Anode: Composition: 30% wt. of CGO-20+70% wt of LSCFO-80     -   Thickness: ˜500 μm.

CGO-20 powder was prepared as from Example 1, a).

The anode was prepared as from Example 3, a) starting from 0.21 g of CGO-20 and 0.49 g of LSCFO-80 (Praxair) which correspond to a 30:70% wt.

The electrolyte membrane was prepared as from Example 3, b).

The cathode was prepared as from Example 3, c) starting from 1 g of LSCFO-80 (Praxair).

The cell evaluation was carried out as described in Example 1, d). The results are set forth in FIG. 6. A current density of 6 A/cm² was observed at 0.7 V dc operating voltage.

EXAMPLE 5 LSCFO-CGO-LSMO/CGO (Asymmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: LSCFO-80

-   -   Thickness: ˜20 μm.         Electrolyte membrane: Composition: CGO-20     -   Thickness: 8 μm         Anode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80     -   Thickness: ˜500 μm.

CGO-20 powder was prepared as from Example 1, a).

The anode was prepared as from Example 3, a) starting from 0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) which correspond to a 30:70% wt.

The electrolyte membrane was prepared as from Example 3, b).

The cathode was prepared as from Example 3, c) starting from 1 g of LSCFO-80 (Praxair).

The cell evaluation was carried out as described in Example 1, d). The results are set forth in FIG. 7. A current density of 1.5 A/cm² was observed at 0.7 V dc operating voltage.

EXAMPLE 6 LSMO/CGO-CGO-LSCFO (Asymmetric)

An electrochemical oxygen separator cell with the following structure and composition was prepared and tested:

Cathode: Composition: 30% wt. of CGO-20+70% wt of LSMO-80

-   -   Thickness: ˜500 μm.         Electrolyte membrane: Composition: CGO-20     -   Thickness: 8 μm         Anode: Composition: LSCFO-80     -   Thickness: ˜20 μm.

CGO-20 powder was prepared as from Example 1, a).

The cathode was prepared as from Example 3, a) starting from 0.21 g of CGO-20 and 0.49 g of LSMO-80 (Praxair) which correspond to a 30:70% wt.

The electrolyte membrane was prepared as from Example 3, b).

The anode was prepared as from Example 3, c) starting from 1 g of LSCFO-80 (Praxair).

The cell evaluation was carried out as described in Example 1, d). The results are set forth in FIG. 8. A current density of 0.7 A/cm² was observed at 0.7 V dc operating voltage.

The electrochemical oxygen separator cells of the invention show a current density dramatically higher than that described in the prior documents, obtained at the same voltage. 

1-12. (canceled)
 13. An electrochemical oxygen separator cell comprising: a cathode comprising a material selected from lanthanum strontium manganese oxide/doped ceria in a ratio of 85:15 to 75:25 by weight; and lanthanum strontium cobalt iron oxide; an electrolyte membrane comprising ceria doped from 15 to 25% by mole; and an anode comprising a material selected from lanthanum strontium manganese oxide/doped ceria in a ratio of about 85:15 to about 75:25 by weight; and lanthanum strontium cobalt iron oxide.
 14. The electrochemical oxygen separator cell according to claim 13, wherein the ceria is doped with an oxide selected from gadolinia and samaria.
 15. The electrochemical oxygen separator cell according to claim 13, wherein the ceria is doped at 20% by mole.
 16. The electrochemical oxygen separator cell according to claim 13, wherein the doped ceria of the electrolyte membrane is Ce_(0.8)Gd_(0.2)O_(1.90).
 17. The electrochemical oxygen separator cell according to claim 13, wherein the lanthanum strontium manganese oxide/doped ceria ratio is 80:20 to 70:30 by weight.
 18. The electrochemical oxygen separator cell according to claim 13, wherein the lanthanum strontium manganese oxide is La_(0.8)Sr_(0.2)MnO₃.
 19. The electrochemical oxygen separator cell according to claim 13, wherein the cathode or anode or both comprises lanthanum strontium cobalt iron oxide.
 20. The electrochemical oxygen separator cell according to claim 19, wherein the lanthanum strontium cobalt iron oxide La_(0.6)Sr_(0.4)Fe_(0.8)Co_(0.2)O₃.
 21. The electrochemical oxygen separator cell according to claim 13, wherein the lanthanum strontium cobalt iron oxide is added with doped ceria.
 22. The electrochemical oxygen separator cell according to claim 13, wherein at least one of the cathode and the anode is thicker than the electrolyte membrane.
 23. The electrochemical oxygen separator cell according to claim 22, wherein the anode is thicker than the electrolyte membrane.
 24. An apparatus comprising an electrochemical oxygen separator cell as described in any one of claims 13-23. 